Variable time base generator



July 29, 1958 c. F. ALTHOUSE 2,845,547

" VARIABLE TIME BASE GENERATOR Filed Nov. 17, 1954 Fig. I

IN V EN TOR. CHARLES I". AL 7' HOUSE TTOR/VEYS United States Pateht OThe invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to variable time base generator circuits and moreparticularly to variable time base generator circuits of the blockingoscillator type employing transistors and which produce a continuouslyvariable time base voltage for-a fixed repetition period and having aconstant width insensitive to frequency.

Sweep generators performing this type of operation have been built withvacuum tubes but the problem of obtaining a continuously variable timebase voltage as represented by a linear sawtooth has always beendilficult to overcome and generally several tubes, with resultingincreased power consumption, are required to produce the constant widthas well as the linear sawtooth voltage. Other transistor multivibratorcircuits have been constructed but these do not maintain the necessaryconstant pulse-width when the trigger rate is changed and the resultingwaveforms need additional shaping to be useful. In addition the sawtoothsweep voltage has too long a retrace time and is non-linear. Co-pendingapplication, Serial No. 474,305, filed December 9, 1954, by Arthur J.Glenn for Transistor Mono-Stable'Sweep Generator was designed toovercome these defects by means of a multivibrator arrangement.

The constantly variable time base voltage generator circuit of thepresent invention overcomes the objections found in the prior artthrough the use of a blocking oscillator and a diode pick-01farrangement using junction type transistors. A sawtooth-voltage timebase represent-v ing the full period between trigger pulses is generatedby the blocking-oscillator circuit and an amplitude comparator using adiode allows a portion of the period to be selected. Subsequent gatescan then be set up to pass the desired waveform.

An object of this invention is the provision of a transistorizedblocking oscillator type circuit for use as a time-base generator.

Another object is the provision of a time-base generator which willproduce a continuously variable time base for a fixed repetition period.

Another object is the provision ofa time-base generator for producing alinear sawtooth sweep voltage of constant width insensitive tofrequency. I 7

Another object is the provision .of a time-base generator of simpledesign that is inexpensive in .manufacture and reliable in operation.

Other objects and many of the attendant'advantages of this inventionwill be readily appreciated asthe same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. 1 shows a diagram of the variable time base generator circuit; and

Fig. 2 shows the voltage waveform curves.

Referring now to Fig. 1, junction-type transistor 11 and the associatedcomponents resistor. 12, capacitor 13; and transformer 14, form theblocking oscillator 16. Resistor 12 and capacitor 13 are variable so thepulse ratecan be adjusted over wide limits.

2,845,547 Patented July 29, 1958 The negative triggering impulsewaveform (A in Fig. 2) is fed through condenser 17 to the base oftransistor 11 which causes a pulse to be developed by the blockingoscillator 16 across transformer 14. At the time of the trigger pulse, anegative voltage pulse is developed across the output winding oftransformer 14 and diode 18 conducts. Although not shown, a positivetrigger pulse connected to the emitter of transistor 11 produces thesame negative pulse on the transformer output winding, causing the diodeto conduct. Since the source impedance, i. e., secondary transformerwindings, is quite low, capacitor 19 tends to charge to the peakamplitude of the pulse voltage. Thus the magnitude of the charge is afunction of the pulse amplitude. When the pulse ceases, the capacitor 19is charged negatively. Since parallel discharge paths exist, through theback resistance of diode 18 and through the resistor 21, a diode with ashigh a back re-' sistance possible is used, so that in the practicaloper-.

ation of this circuit, the discharge path back through the diode neednot be considered. The negative charge on condenser 19, which remainsafter the trigger pulse has ended, leaks off through resistor 21 in theother discharge path and is returned to a positive variable voltage. Thevariable positive voltage and resistor 21 connected to this voltage arethe primary timing parameters and the discharge rate of capacitor 19becomes a function of both the value of resistor 21 and the magnitude ofthe By making either of these variable positive voltage. variable, therate of discharge may be controlled and the waveform across capacitor 19is shown as B in Fig. 2 with the dotted lines representing differentvalues of the voltage or resistance 21. When this voltage reaches groundpotential, rising from its negative value because of the capacitorcharge following the pulse, it is shortcircuited by the diode 18, whichis again conducting since the anode is now near Zero potential. Thevoltage now remains at ground potential until the next pulse occurs. Itis readily apparent that the sloping portion is com stant inamplitude'but variable in slope. This rising slope portion is most casesis the useful portion of the waveform, but must be converted to awaveform shaped similar to E in Fig. 2 to be usable for currentpractical equipment design. The remaining circuitry in this invention isfor the purpose of deriving a waveform of shape D to be added towaveform B tov obtain the resultant desired waveform E.

A low value of resistance 22 is inserted as shown so that charge anddischarge currents will flow through it, developing a voltage waveformshown as C in Fig. 2. It is necessary to detect these points of change,and particularly the discharge period, which is done by successivestages] of amplicatio-n. The resistor 22 is connected directly to theemitter of a grounded-base junction transistor 23. The function ofresistance 22 and the low resistance of the emitter to base oftransistor 23 is to detect the period of dischargeof the condenser 19.Since the rate of discharge is constant, the current is constant, sothat a positive voltage across resistor 22 exists during the discharge]period. This voltage is amplified by transistor 23 which is connected asa grounded-base amplifier. The resistor 24 connected to theemitter oftransistor 23 is merely a biasing resistance to ensure that the emittercurrent is proper for the operation of transistor 23 as an amplifier.

Because the time period can vary over suchwide limits, and constantoperating levels should be sustained,

it is necessary to directly couple the output of transistor 23 acrossresistor 26 to transistor 27. This is done. through the network composedof resistor 28 and capacitor 29. This capacitor serves primarily toby-pass the high frequency components of the rectangular waveform ofvoltage across load resistor'26 direct to the input of." ilsnsistor 27,whose collector is connected to'ground,"

and which functions as an isolation amplifier with no gain.

The output voltage developed across load resistor 31 is connected to theemitter of transistor 32 through the coupling network consisting ofresistor 33 and capacitor 34. This capacitor serves to pass the highfrequency components to the transistors emitter. The transistor 32 isconnected in a conventional grounded-base amplifier arrangement whoseoutput is developed across resistor 36 and the equivalent resistance ofthe collector-base of transistor 37. Transistor 37 is connectedessentially as a high-gain amplifier isolation stage whose output isdeveloped across resistor 38, whose waveform is essentially rectangularas indicted by D in Fig. 2. This is the desired gating voltage which isapplied to the emitter of transistor 39 whose collector is connected toground. Capacitor 41 couples the output of transistor 37 to transistor23 in a regenerative feedback loop to sharpen up the wavefront. Thus,when waveform B reaches ground potential, the high current through thediode and transformer windings and through resistor 21 causes a voltageto be developed which is further increased in magnitude by theconnection of this feedback loop.

Transistor 39 is a second isolation amplifier stage whose collector isconnected to ground. It serves to couple the variable amplitude ofwaveform D to the mixing transistor 42. The waveform B is connected tothe base of transistor 42 and essentially a portion of waveform D isconnected to the emitter. The mixed output E now results across resistor43. This is the desired time base, rising from a Zero level as apositive sawtooth voltage and returning to it at the end of a desiredperiod. The waveform D is useful as a gating function in related circuitoperations where such a device might be employed.

This circuit offers several advantages in that the pedestal upon whichthe time base exists can be controlled, so that some advantage can beachieved in developing fast time bases. Another advantage of thiscircuit is that it offers a continuously variable voltage time baseoperation over the full range of the triggeringpulse period. It issensitive to frequency and offers excellent possibilities for linearcalibration scales.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. In a variable time base generator, circuit means for producing avoltage going sharply negative with respect to ground upon applicationof a triggering voltage pulse thereto, and a linear rise back to groundpotential, at which the voltage remains until the next triggering pulseis applied, said circuit means comprising a transitorized blockingoscillator including a transformer, a charge-discharge condenser,normally conducting gating means connected to pass negative charges fromthe secondary winding of said transformer to one plate of saidcharge-discharge condenser, a variable resistor connecting a positivepotential source to said one plate, and an output connection betweensaid gating means and said one plate.

2. In a variable time base generator, circuit means for producing avoltage going sharply negative with respect to ground upon applicationof a triggering voltage pulse thereto, and a linear rise back to groundpotential, at which the voltage remains until the next triggering pulseis applied, said circuit means comprising a junction type transistor, aresistance connecting a positive potential source to the emitter thereofand a capacitor connecting said emitter to ground, a transformer, theprimary winding of said transformer connecting the base of saidtransistor to ground, an intermediate winding on said trans formerconnecting the collector of said transistor to ,a.

negative potential source, a normally conducting diode, the secondarywinding of said transformer connecting the diode to ground, acharge-discharge condenser, one plate of which is connected to saiddiode, a variable resistor connecting said one plate to a secondpositive potential and an output connection between said diode and saidone plate.

3. The circuit as in claim 2, in which the resistor and capacitorconnected to said emitter are adjustable to permit a wide range in thepulse repetition rate.

4. The circuit as in claim 2, in which said diode has a high reverseresistance such that the negative charge on said one plate is dischargedthrough the path of said variable resistor, said variable resistor beingadjustable to determine the desired discharge rate of saidcharge-discharge condenser.

5. A triggered sawtooth generator comprising a capacitor, a source ofpositive potential, a resistor connccting one side of said capacitor tosaid source, means for feeding a negative trigger pulse to said one sideof said capacitor to store a negative charge therein for lineardischarge through said resistor, and means included in said feedingmeans and etfective following said discharge and prior to receipt of asecond trigger pulse for maintaining the voltage at said one side ofsaid capacitor at a value not greater than a predetermined po tentialwhich is less than the potential of said source.

6. The generator of claim 5 wherein said last mentioned means and saidtrigger pulse feeding means comprise a normally conducting diode havingthe anode thereof connected to said one side of said capacitor, thecathode of said diode having a source of potential lower than said firstmentioned source, whereby as the capacitor discharges toward saidpositive potential after reception of a trigger pulse the diode willconduct when the voltage at said capacitor reaches the potential of saidcathode.

7. In a variable time base generator triggered by a train of pulses,means responsive to said train and including a series connectedcapacitor and resistor for generating across said resistor and at therepetition rate of said pulses a sawtooth voltage of constant amplitudehaving a predetermined slope time, means including a grounded-basejunction transistor having said resistor connected to the emitterthereof for detecting said slope time, means responsive to saiddetecting means connected to the collector of said transistor and forgenerating a rectangular'gating voltage having a duration equal to saidslope time, and means connected to the output of said generating meansfor mixing said sawtooth voltage and said gating voltage.

8. The structure of claim 7 wherein said mixing means comprises a secondtransistor having an emitter, base and grounded collector, saidcapacitor coupling the base of said second transistor with the emitterof said transistor. said gating voltage generating means being coupledbetween the collector of said first mentioned transistor and the emitterof said second transistor.

9. In a device of the class described, a capacitor, rcsistor, means foralternately charging and discharging: said capacitor, through saidresistor and means connected to said resistor and capacitor fordetecting the discharge time of said capacitor, said detecting meanscomprising a transistor having an emitter, collector and base, saidresistor being connected in the emitter-base circuit of said transistor.

References Cited in the file of this patent UNITED STATES PATENTS2,258,752 Fewings et al. Oct. 14, 1941 2,620,448 Wallace Dec. 4, 19522,745,012 Felker May 8, 1956 FOREIGN PATENTS 144,789 Australia Jan. 17,1952

